Method and apparatus for assigning codes

ABSTRACT

A base station controller selects codes corresponding to rate information included in a new starting call message from a mobile station, and further selects codes assignable to the mobile station out of the selected codes as candidate codes. Then, the base station controller detects codes which meet both the characteristics of being at upper level of the candidate codes in the tree structure and being assignable to other mobile station, and further detects levels of the detected codes. Detected levels for all the candidate codes are compared to find a candidate code whose level is the lowest and to assign the candidate code to the mobile station sending the new starting call message.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to code assignment in multi-ratecommunications of the radio communication system employing Code DivisionMultiple Access (CDMA) system, where codes having tree structuredorthogonal multi-spreading factor sequence are used as spreading codes.

[0003] 2. Description of the Related Art

[0004] The system utilizing coherent multi code and the system utilizingtree structured multi-spreading factor sequence code have beenintroduced as a multi-rate transmission system in the radiocommunication system employing DS-CDMA (Direct Sequence-CDMA). In themulti-rate transmission utilizing the coherent multi code, plural codesare simultaneously used to perform multi-code-multiplexing. In themulti-rate transmission utilizing the tree structured multi-spreadingfactor sequence code, orthogonal codes having equivalent spreadingfactors are multiplexed in parallel. Receivers for receiving pluralcodes are needed for a mobile station used for the multi-ratetransmission utilizing the coherent multi code. On the other hand, onlyone receiver is needed for the system utilizing the tree structuredmulti-spreading factor sequence code.

[0005] The conventional CDMA system using tree structured orthogonalmulti-spreading factor sequences is disclosed in, for example, atechnical report entitled “Multi-rate Wideband DS-CDMA Radio Access ForNext Generation Mobile Radio Systems”, the Institute of Electronics,Information and Communication Engineers, RCS-97-86, 1997. Theconventional system will be explained with reference to FIG. 11.

[0006]FIG. 11 shows a configuration of the conventional tree structuredorthogonal multi-spreading factor sequences.

[0007] The tree structure is composed of four levels: level 1 at the topthrough level 4 at the bottom, defined just for convenience. As thereare four levels in this case, the channel transmission rate using thetop level (level 1) code is eight times as fast as that of the channelusing the lowest level (level 4) code. The channel transmission rateusing level 2 code is four times the rate of channel using the level 4code. The channel transmission rate using level 3 code is twice the rateof channel using the level 4 code. Cx(y) in FIG. 11 represents a kind ofcodes, x stands for a code level, y stands for an identification numberin the same code level, and .smallcircle. stands for a code.

[0008] In FIG. 11, a code in the level 1 can be used for communicationof twice the transmission rate of level 2. Now, a concrete example willbe explained referring to the case of tree structure as shown in FIG.11. When C.sub.2(1) is assigned to a user of four times the transmissionrate of the lowest rate (C.sub.4(x) level in FIG. 11), the codesC.sub.4(1), C.sub.4(2), C.sub.4(3), C.sub.4(4), C.sub.3(1), andC.sub.3(2) can not be assigned to other users because of keeping thecode orthogonality. Due to this characteristic, all the lower levelcodes connected with an assigned code through branches and all the upperlevel codes connected with the assigned code through only upwardbranches are not-assignable. When C.sub.2(1) is the assigned code, allthe lower codes C.sub.4(1), C.sub.4(2), C.sub.4(3), C.sub.4(4),C.sub.3(1) and C.sub.3(2) connected with the assigned code by branchesare not-assignable, and the upper code C.sub.1(1) connected with theassigned code by the shortest branch is not-assignable, either.

[0009] As stated above, in CDMA system using codes of tree structuredorthogonal multi-spreading factor sequences, the code assignment for amobile station is restricted because of the characteristic. Accordingly,in the case of FIG. 11, if C.sub.4(5) and C.sub.4(7) are assigned tocalls of the lowest rate (level 4), it is impossible to assign a callhaving the rate of level 3 or upper than level 3. However, on thesupposition that C.sub.4(6) is assigned to the call of level 4 insteadof C.sub.4(7), it is possible to assign C.sub.3(4) to the call of level3, consequently channels are effectively utilized.

OBJECTS OF THE PRESENT INVENTION

[0010] In the multi-rate transmission of the conventional radiocommunication system employing Code Division Multiple Access (CDMA)system where codes having tree structured orthogonal multi-spreadingfactor sequence are used, when a code has been already assigned (inuse), it is impossible to assign all the codes upper than the code. Thiscauses a problem that the channel utilization is deteriorated. It is anobject of the present invention to utilize all the system (all thebranches) of the tree structure for assigning a code to be responsive toa new call, in order to enhance the channel utilization.

[0011] In the conventional art, a channel mismatch would occur at thecall ending, even if a channel match was kept at the code assigning timeof a new call starting. This sometimes causes a problem that selecting acode to be assigned to a new call is impossible. It is another object ofthe present invention is to avoid the channel mismatch by switching acode being used in communication. Consequently, call loss rate can bereduced.

[0012] In the code assignment of the conventional radio communicationsystem employing CDMA system, it is necessary for a base station and amobile station to simultaneously switch codes in order not to stop thecommunication under way. However, as the base station does not havemeans for judging a code switch timing over the mobile station, the basestation is troublesomely needed to simultaneously perform reception andtransmission using both the codes used before and after switching. It isanother object of the present invention to reduce the stop time causedby code switching and the hardware needed for reception/transmissionusing both the codes between the base station and the mobile station, bymeans of sending the code switching timing from the base station.

[0013] In the code assignment of the conventional radio communicationsystem employing CDMA system, there is a problem that much process isneeded for synchronizing the code switch timings in the base station andthe mobile station. It is another object of the present invention toreduce the code switching process by means of depending on mobilestation's own timing.

[0014] Namely, it is a general object of the present invention toprovide an apparatus and a method for effectively utilizing the channel,comprising means in the base station to judge a code switch timing overthe mobile station, and reducing the synchronization process.

SUMMARY OF THE INVENTION

[0015] According to one aspect of the present invention, a codeassigning apparatus used in a radio communication system including abase station controlling apparatus, a plurality of base stations, and aplurality of mobile stations, employing CDMA (Code Division MultipleAccess) method, which uses codes of tree structured orthogonalmulti-spreading factor sequences as spreading codes, for radio accessbetween the plurality of base stations and the plurality of mobilestations, and providing multi-rate transmission between the plurality ofmobile stations and the plurality of base stations, the code assigningapparatus comprises

[0016] a code selecting unit for selecting an assignable code whichsuits specifications requested by a new starting call and exists in alocation as close to a not-assignable code as possible and in a levellower than the not-assignable code in the tree structure.

[0017] According to another aspect of the present invention, the codeselecting unit of the code assigning apparatus comprises

[0018] a first selecting unit for selecting one or more than one codescorresponding to rate information included in a message from one of theplurality of mobile stations, as first codes,

[0019] a second selecting unit for selecting one or more than one firstcodes if they are assignable to the one of the plurality of mobilestations, as second codes,

[0020] a level detecting unit for retrieving codes upper than the secondcodes one by one, detecting not-assignable codes, which are assigned toanother of the plurality of mobile stations, out of the codes upper thanthe second codes, as third codes, and detecting each level of the thirdcodes,

[0021] a third selecting unit for selecting one of the second codeswhose upper third code is in a lowest level in the tree structure, and

[0022] a code assigning unit for assigning the one of the second codesselected by the third selecting unit to the one of the plurality ofmobile stations.

[0023] According to another aspect of the present invention, the codeassigning apparatus comprises

[0024] a code-switching/code-assignment processing unit,

[0025] when there is no assignable code which suits specificationsrequested by a new starting call, for making a code which isnot-assignable and suits the specifications requested by the newstarting call assignable by way of switching an assigned code being usedto another code connected by a branch different from a branch used forthe assigned code in the tree structure.

[0026] According to another aspect of the present invention, thecode-switching/code-assignment processing unit of the code assigningapparatus comprises

[0027] a first selecting unit for selecting one or more than one codescorresponding to rate information included in a message from the secondmobile station, as first codes,

[0028] a second selecting unit for selecting one or more than one firstcodes if they are assignable to the second mobile station, as secondcodes,

[0029] a fourth selecting unit, when there is no second code, forselecting one or more than one first codes if their lower codes havebeen assigned to the first mobile station, as fifth codes,

[0030] an assigned code detecting unit for detecting a code lower thanthe fifth codes and assigned to the first mobile station as a fourthcode,

[0031] a switchable code detecting unit for detecting a code to whichswitchable from the fourth code, for the first mobile station, as asixth code,

[0032] a code switch directing unit for directing the first mobilestation to switch from the fourth code to the sixth code, and

[0033] an assigning unit for assigning a fifth code to the second mobilestation after switching all the fourth codes to other codes.

[0034] According to another aspect of the present invention,

[0035] the code-switching/code-assignment processing unit includes acode switch informing unit for informing that the fourth code havingbeen used by the first mobile station can be switched to a sixth code,

[0036] one of the plurality of base stations includes a timinginformation sending unit for sending timing information of switchingfrom the fourth code to the sixth code to the first mobile station,

[0037] the first mobile station includes a code switching unit forswitching the fourth code to the sixth code based on the informing ofthe code-switching/code-assignment processing unit and the timinginformation of the one of the base stations,

[0038] the one of the plurality of base stations includes a switchingunit for switching downlink transmission codes at time when the timinginformation is sent to the first mobile station, and thecode-switching/code-assignment processing unit includes

[0039] a code releasing unit for releasing the fourth code used by thefirst mobile station, at the time when the timing information is sent tothe first mobile station.

[0040] According to another aspect of the present invention, thecode-switching/code-assignment processing unit includes a first codeswitching unit for informing the first mobile station that a fourth codebeing used by the first mobile station is to be switched to a sixthcode,

[0041] one of the plurality of base stations includes a transmittingunit for performing transmission using both the fourth code and thesixth code to the first mobile station,

[0042] the first mobile station includes a regenerating unit forreceiving both the fourth code and the sixth code and generatingreception information by performing maximum ratio combination, a secondcode switching unit for switching a state of synthesizing maximum ratioto a state of receiving only the sixth code based on own timing, and aswitching complete informing unit for informing that code switching hasbeen completed in the first mobile station, and

[0043] the code-switching/code-assignment processing unit includes acode releasing unit for releasing the fourth code used by the firstmobile station, based on the informing from the first mobile station.

[0044] According to another aspect of the code assigning apparatus ofthe present invention, the code selecting unit is provided in either oneof the plurality of base stations or the base station controlling unit.

[0045] According to another aspect of the code assigning apparatus ofthe present invention, the code-switching/code-assignment processingunit is provided in either one of the plurality of base stations or thebase station controlling unit.

[0046] According to one aspect of a method of assigning codes of thepresent invention, used in a radio communication system including a basestation controlling apparatus, a plurality of base stations, and aplurality of mobile stations, employing CDMA (Code Division MultipleAccess) which uses codes of tree structured orthogonal multi-spreadingfactor sequences as spreading codes, for radio access between theplurality of base stations and the plurality of mobile stations, andproviding multi-rate transmission between the plurality of mobilestations and the plurality of base stations, the method of assigningcodes comprises the steps of

[0047] selecting one or more than one codes corresponding to rateinformation included in a message from one of the plurality of mobilestations, as first codes,

[0048] selecting one or more than one first codes if they are assignableto the one of the plurality of mobile stations, as second codes,

[0049] retrieving codes upper than the second codes one by one,

[0050] detecting not-assignable codes, which are assigned to another ofthe plurality of mobile stations, out of the codes upper than the secondcodes, as third codes,

[0051] detecting each level of the third codes,

[0052] selecting one of the second codes whose upper third code is in alowest level in the tree structure, and

[0053] assigning the one of the second codes selected by the above stepto one of the plurality of mobile stations.

[0054] According to another aspect of the method of assigning codes ofthe present invention comprises the steps of

[0055] selecting one or more than one codes corresponding to rateinformation included in a message from the second mobile station asfirst codes,

[0056] selecting one or more than one first codes if they are assignableto the second mobile station as second codes,

[0057] when there is no second code, selecting one or more than onefirst codes if their lower codes have been assigned to the first mobilestation, as fifth codes,

[0058] detecting a code lower than the fifth codes and assigned to thefirst mobile station, as a fourth code,

[0059] detecting a code to which switchable from the fourth code for thefirst mobile station, as a sixth code,

[0060] directing the first mobile station to switch from the fourth codeto the sixth code, and

[0061] assigning a fifth code to the second mobile station afterswitching all the fourth codes to other codes.

[0062] According to another aspect of the method of assigning codes ofthe present invention comprises the steps of

[0063] informing that a fourth code having been used by the first mobilestation can be switched to a sixth code,

[0064] sending timing information of switching from the fourth code tothe sixth code to the first mobile station,

[0065] switching the fourth code to the sixth code based on theinforming of the informing step and the timing information of thesending timing information step,

[0066] switching downlink transmission codes based on the timinginformation sent to the first mobile station, and

[0067] releasing the fourth code used by the first mobile station, basedon the timing information.

[0068] According to another aspect of the method of assigning codes ofthe present invention comprises the steps of

[0069] informing the first mobile station that a fourth code being usedby the first mobile station is to be switched to a sixth code,

[0070] performing downlink transmission using both the fourth code andthe sixth code to the first mobile station,

[0071] receiving both the fourth code and the sixth code,

[0072] generating reception information by performing maximum ratiocombination,

[0073] switching a state of synthesizing maximum ratio to a state ofreceiving only the sixth code based on own timing,

[0074] informing that code switching has been completed in the firstmobile station, and

[0075] releasing the fourth code used by the first mobile station, basedon the informing from the first mobile station.

[0076] The above and other objects and features of the invention willappear more fully hereinafter from a consideration of the followingdescription taken in connection with the accompanying drawing whereinone example is illustrated by way of example.

BRIEF DESCRIPTION OF THE DRAWINGS

[0077] In the drawings:

[0078]FIG. 1 shows a configuration of radio communication systemapplying a code assignment method according to the present invention;

[0079]FIG. 2 illustrates sequence of a code assignment procedureaccording to the present invention;

[0080]FIG. 3 shows a code structure used for the code assignmentprocedure of FIG. 2;

[0081]FIG. 4 is a flowchart showing a process of the code assignmentprocedure of FIG. 2;

[0082]FIG. 5 illustrates sequence of another code assignment procedureaccording to the present invention;

[0083]FIG. 6 shows a code structure used for a code switching procedureof FIG. 5;

[0084]FIG. 7 shows a code structure used for selecting a code-switchingcandidate code;

[0085]FIG. 8 is a flowchart showing a process of the code assignmentprocedure of FIG. 5;

[0086]FIG. 9 illustrates sequence of another code assignment procedureaccording to the present invention;

[0087]FIG. 10 illustrates sequence of another code switching procedureaccording to the present invention;

[0088]FIG. 11 shows a tree structure of codes having tree structuredorthogonal multi-spreading factor sequence, used in a conventional art;

[0089]FIG. 12 shows a configuration of a code selecting unit in a basestation controller according to the present invention;

[0090]FIG. 13 shows a level list according to the present invention;

[0091]FIG. 14 shows configurations of a code selecting unit and acode-switching/code-assignment processing unit, in a base stationcontroller according to the present invention;

[0092]FIG. 15 shows a switching list according to the present invention;

[0093]FIG. 16 shows a system structure according to the presentinvention; and

[0094]FIG. 17 shows a system structure according to the presentinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0095] Embodiment 1.

[0096]FIG. 1 shows a configuration of radio communication systemapplying a code assignment method according to the present embodiment.FIG. 2 illustrates sequence of a code assignment procedure according tothe present embodiment. FIG. 3 shows a code structure example of thecode assignment procedure of FIG. 2. FIG. 4 is a flowchart showing aprocess of the code assignment procedure of FIG. 2. FIG. 12 shows aconfiguration of a base station controller 1. Referring to FIGS. 1, 2,3, 4, and 12, one embodiment of code assignment performed in a basestation 2 and the base station controller 1 according to the presentinvention will be described.

[0097]FIG. 1, showing a configuration of radio communication system ofthe present embodiment and other embodiments, is composed of pluralmobile stations 3 a, 3 b and 3 c, plural base stations 2 a, and 2 b, andthe base station controller 1 which controls the plural base stations 2a and 2 b. The mobile stations 3 a, 3 b and 3 c are just called as a“mobile station 3” in the case of describing each mobile station beingunnecessary. The base stations 2 a and 2 b are also called as a “basestation 2” in the case of describing each base station beingunnecessary. The mobile station 3 is connected to the base station 2 viaa radio channel. The radio channel utilizes the Code Division MultipleAccess (CDMA) system as a radio access system, and uses spreading codeshaving tree structured orthogonal multi-spreading factor sequences.According to the radio communication system of the present embodiment,it is possible to set up multiple rates. Namely, the base station 2 andthe mobile station 3 can set up channels of plural-kinds rates for eachcall, using the radio channel. As orthogonal multi spreading codes areused, a channel having a double transmission rate can be set up inproportion as a code level goes up one by one.

[0098]FIG. 2 illustrates sequence of a code assignment procedure inresponsive to a call from the mobile station 3. Referring to FIGS. 1 and2, the code assignment procedure regarding the base station controller1, the base station 2 and the mobile station 3 will now be explained.First, a radio channel set-up request message is transmitted from themobile station 3 to the base station controller 1 via the base station2. The base station controller 1 analyzes the message in order to detecta transmission rate requested by the mobile station 3. The base stationcontroller 1 determines a pair of codes for transmission to be used bythe mobile station 3 and the base station 2, based on the codeassignment process stated later. Then, the base station controller 1transmits the radio channel set-up request message to the base station2.

[0099] The radio channel set-up request message includes information ofthe code pair. Receiving the message, the base station 2 begins toprepare for reception/transmission based on an appointed code, andtransmits a radio channel set-up complete message to the base stationcontroller 1. On receiving the radio channel set-up complete message,the base station controller 1 transmits the radio channel set-upcomplete message appointing the pair of codes to the mobile station 3.After receiving the message, the mobile station 3 starts communicationwith the base station 2, based on the appointed code.

[0100] As stated above, the communication channel between the mobilestation 3 and the base station 2 is set up based on the request from themobile station 3. A code selection process, performed in the basestation controller 1 for the mobile station 3 and the base station 2 isexplained with reference to FIGS. 3, 4 and 12. FIG. 3 shows a treestructure of codes having orthogonal multi-spreading factor sequence. InFIG. 3, the tree structure is composed of four levels; level 1 at thetop through level 4 at the bottom, defined just for convenience. Asthere are four levels in this case, the channel transmission rate usingthe top level (level 1) code is eight times as fast as that of channelusing the lowest level (level 4) code. The channel transmission rateusing level 2 code is four times the rate of channel using level 4 code.The channel transmission rate using level 3 code is twice the rate ofchannel using level 4 code.

[0101] Cx(y) in FIG. 3 represents a kind of codes; x stands for a codelevel, y stands for an identification number in the same code level, and.smallcircle. stands for a code. In the tree structured orthogonal multispreading sequence codes, codes lower than a specific code being used(that is, larger number level codes connected with the specific code bybranches in FIG. 3) can not be used because of keeping the orthogonalitycharacteristic.

[0102] In addition, codes upper than the specific code being used (thatis, smaller number level codes connected with the specific code throughonly upward branches) also can not be utilized. When C.sub.3(2) is beingused in FIG. 3, C.sub.1(1), C.sub.2(1), C.sub.4(3) and C.sub.4(4) cannot be used. Therefore, it is necessary to appropriately assign a codeto a new call from the mobile station 3.

[0103]FIG. 12 shows a configuration of a code selecting unit 10 in thebase station controller 1. FIG. 4 is a flowchart showing code assignmentprocesses, for the mobile station 3 and the base station 2, performed inthe code selecting unit 10. When a first selector 11 of the base stationcontroller 1 receives the radio channel set-up request message from themobile station 3 (S0101), the first selector 11 analyses rateinformation in the message (S0102), and determines a code level neededfor the mobile station 3 and the base station 2 (S0103). In FIG. 3, forinstance, the code level requested by the mobile station 3 is level 3.Secondly, a second selector 12 of the base station controller 1 selectscandidate codes which can be used in the determined level (S0104). InFIG. 3, C.sub.3(1), C.sub.3(3) and C.sub.3(4) are selected as thecandidate codes.

[0104] A level detector 13 of the base controller 1 selects a code outof the candidate codes (S0105), selects another code in a level one moreupper than the selected code (S0106), and judges whether or not the onemore upper level code can be assigned (S0107). In FIG. 3, if thecandidate code is C.sub.3(1), the one more upper level code isC.sub.2(1). Then, when the one more upper level code is able to beassigned, the judging procedure is repeated upwards along the branchesuntil it comes to a code which can not be assigned (S1018, S0109). If acode that is not assignable is detected, the code and its level arememorized in a level list 16 for each of the candidate codes (S0107,S0110). When C.sub.3(1) in FIG. 3 is the candidate code, one more uppercode C.sub.2(1) is not assignable. Then, C.sub.3(1), C.sub.2(1) and thelevel 2 are stored in the level list 16 as shown in FIG. 13. Thisprocedure is performed for each candidate code to complete the levellist 16 shown in FIG. 13 (S0111).

[0105] A third selector 14 selects a candidate code whose uppernot-assignable code has the lowest level, in the level list 16 (S0112).Then, the candidate code is selected to be assigned to the mobilestation 3 (S0113). If there are several candidate codes whose uppernot-assignable codes have the same code level, any of the candidatecodes can be selected. If a candidate code has no upper not-assignablecode, the top level code is regarded as the upper not-assignable code.As shown in FIG. 13, C.sub.2(1) being the upper not-assignable code andlevel 2 are selected for the candidate code C.sub.3(1) in FIG. 3.C.sub.1(1) and level 1 are for the candidate code C.sub.3(3), andC.sub.1(1) and level 1 are for the candidate code C.sub.3(4). Therefore,C.sub.3(1), whose upper not-assignable code is C.sub.2(1), is selectedas a code to be assigned. According to this assigning method, C.sub.3(3)and C.sub.3(4) in FIG. 3 are not selected. The code C.sub.3(1) isassigned to the mobile station 3 by a code assigning unit 15. If a newcall corresponding to level 2 is requested after the call correspondingto level 3 has been in use as shown in FIG. 3, C.sub.2(2) can beassigned to the new call.

[0106] According to the present assigning method, a channel mismatch atthe code assigning time of a new call starting, caused by the treestructure characteristic, is avoided. Therefore, a code assignment formulti-rate transmission can be performed without deteriorating thechannel utilization efficiency. It is also acceptable to provide thecode selecting unit 10 of FIG. 12 in the base station 2 instead of thebase station controller 1.

[0107] Embodiment 2.

[0108]FIG. 5 illustrates a code assignment sequence showing a codeswitching procedure, according to the present embodiment. FIG. 6 shows acode structure applied for the code switching procedure of FIG. 5. FIG.7 shows a code structure used in selecting a candidate code for the codeswitching. FIG. 8 is a procedure flowchart for the code assignmentsequence of FIG. 5. FIG. 14 shows a configuration of the base stationcontroller according to the present embodiment. Referring to FIGS. 1, 5,6, 7, 8 and 14, the code assignment according to the present embodimentwill now be described.

[0109] The code switching is explained with reference to FIG. 6 showinga code assignment state change. The same code definition as FIG. 3 isused in FIG. 6. As stated in Embodiment 1, when a code has been alreadyassigned to the mobile station 3, it is impossible to newly assign acode in upper level than the assigned code to another mobile station 3,because of the characteristic of spreading codes having tree structuredorthogonal multi-spreading factor. In the left of FIG. 6, as C.sub.4(1)and C.sub.4(3) have been already assigned, it is impossible to assignC.sub.3(1) and C.sub.3(2) to another mobile station 3. Accordingly, ifthere is a call requesting a transmission rate corresponding to level 3at this time, the call is to be lost. By switching the code fromC.sub.4(3) to C.sub.4(2), however, C.sub.3(2) can be assigned, so thatthe call requesting the transmission rate corresponding to level 3 isaccepted.

[0110] Referring to FIGS. 1 and 5, the code assignment procedure and thecode switching procedure regarding the base station controller 1, thebase station 2, and the mobile station 3 will now be explained. In FIG.1, a mobile station #1 (3 a) has been already used, and a mobile station#2 (3 b) newly makes a call. As shown in FIG. 5, the mobile station #2(3 b) transmits a radio channel set-up request message indicating atransmission rate, to the base station controller 1 as stated inEmbodiment 1.

[0111] When there is no assignable code as shown in the left of FIG. 6,it is necessary to perform code switching. Therefore, the mobile station#1 (3 a) and a code are selected for the code switching, based on amethod stated later. The radio channel set-up request message istransmitted to a base station 2 a in order to assign the selected codeto the mobile station #2 (3 b).

[0112] The base station 2 a judges whether the code appointed in theradio channel set-up request message can be used or not. In this case,as the code is being used by the mobile station #1 (3 a), it is judgedto be not usable. Then, a code switching request is transmitted to thebase station controller 1. On receiving the code switching request fromthe base station 2 a, the base station controller 1 transmits a codeswitching request message including information of code after switchingto the mobile station #1 (3 a). In the case of FIG. 6, the base stationcontroller 1 defines C.sub.4(2) as the code after-switching in the codeswitching request message.

[0113] When the mobile station #1 (3 a) receives the code switchingrequest message, the mobile station #1 (3 a) switches the original codeto the target code (code after switching) (C.sub.4(2) in FIG. 6), andtransmits a code switching complete message to the base stationcontroller 1. The base station controller 1 judges, by receiving thecode switching complete message, that the code has been switched in themobile station #1 (3 a). (The state has been changed from the left sideto the right side in FIG. 6.) Then, the base station controller 1transmits the code switching complete message to the base station 2 a.After receiving the code switching complete message, the base station 2a transmits a radio channel set-up complete message to the base stationcontroller 1. The base station controller 1 transmits the radio channelset-up complete message to the mobile station #2 (3 b) in order toassign an assignable and requested-level code whose level iscorresponding to the transmission rate requested by the mobile station#2 (3 b) (that is, C.sub.3(2) in FIG. 6).

[0114] By performing the above procedures, a new radio channel requestedby the mobile station 3 has been set up between the mobile station 3 andthe base station 2. Now, the code switching procedure, for the mobilestation #1 (3 a) and the base station 2 a, performed in the base stationcontroller 1, and the code assigning procedure, for the mobile station#2 (3 b) and the base station 2 a, performed in the controller 1 areexplained with reference to FIGS. 7, 8 and 14. Similar to FIG. 3, FIG. 7shows a tree structure of codes having orthogonal multi-spreading factorsequence. The candidate code/not-assignable in the figure represents anunused code which is corresponding to a requested transmission rate andwhose lower level code has been already assigned. FIG. 14 shows aconfiguration of a code-switching/code-assignment processing unit 20 inthe base station controller 1. FIG. 8 is a flowchart showing a codeswitching and a code assigning process performed in thecode-switching/code-assignment processing unit 20.

[0115] Similar to Embodiment 1, when the first selector 11 of the basestation controller 1 receives the radio channel set-up request messagefrom the mobile station #2 (3 b) (S0101), the first selector 11 analyzesrate information in the message (S0102) and determines a code level inthe tree structure corresponding to the rate information (S0103). InFIG. 7, for instance, the code level corresponding to the rateinformation is level 3. The second selector 12 of the base stationcontroller 1 retrieves an assignable code of the determined level(S0104). When there is no assignable candidate code of the determinedlevel (S0201), a fourth selector 21 selects a code which is notassignable and has not been itself assigned, as a candidate code(S0204). In FIG. 7, C.sub.3(1) and C.sub.3(2) are such codes. In thecase of there being no code which is not assignable and has not beenitself assigned yet (S0202: No), the original call becomes lost (S0203).

[0116] An assigned code detector 22 selects a code out of candidatecodes (C.sub.3(2) in FIG. 7) (S0205), and detects an assigned code inlower level than the selected candidate code. (In FIG. 7 case,C.sub.4(3)) (S0206). A switchable code detector 23 retrieves codes tosee whether there is a switchable code at the same level as the aboveassigned code or not. (S0208). When there is a switchable code in thesame level as the assigned code (C.sub.4(2) in FIG. 7), the candidatecode (C.sub.3(2)), the assigned code (C.sub.4(3)) and the switchablecode (C.sub.4(2)) are stored in a switching list 26 as shown in FIG. 15(S0210). This procedure is performed for every candidate code and everyassigned code at the lower level than the candidate code. (S0207, S0209,S0211). In the case of there being candidate codes, the switchable codedetector 23 selects one candidate code having the least number ofassigned codes at its lower level, as an assignment target code (S0213).

[0117] In the above case, all the assigned codes have their switchablecodes in the same level. In FIG. 7, there are two candidate codes:C.sub.3(1) and C.sub.3(2). As shown in FIG. 15, either of C.sub.3(1) andC.sub.3(2) has one assigned code (C.sub.4(1) is for C.sub.3(1), andC.sub.4(3s) is for C.sub.3(2)), and both the candidate codes C.sub.3(1)and C.sub.3(2) have their switchable codes (C.sub.4(4) for C.sub.3(1)and C.sub.4(2) for C.sub.3(2)) in the same level (level 4). Therefore,either of C.sub.3(1) and C.sub.3(2) can be selected as the assignmenttarget code.

[0118] A code switch directing unit 24 switches the assigned code at thelower level of the selected assignment target code to the storedswitchable code (S0214). In the case of selecting C.sub.3(2) as theassignment target code in FIG. 7, C.sub.4(3) is switched to C.sub.4(2).The code used by the mobile station #1 (3 a) in FIG. 5 is correspondingto the assigned code. An assigning unit 25 assigns the selectedassignment target code to the mobile station #2 (3 b) (S0215).

[0119] The switching possibility is checked for all the assigned codesat the lower level of all the candidate codes, in the presentembodiment. The checking process, however, can be completed at the firsttime of detecting a candidate code which has an assigned code to beswitched at the lower level.

[0120] Even when a channel mismatch exists, according to the presentembodiment, the call loss rate is reduced because a new high rate callcan be achieved by switching a code being used in the mismatch channel.It is also acceptable to provide the code selecting unit 10 and thecode-switching/code-assignment processing unit 20 of FIG. 14 in the basestation 2 instead of the base station controller 1.

[0121] Embodiment 3.

[0122]FIG. 9 illustrates a code assignment sequence showing a codeswitching procedure, according to the present embodiment. FIG. 16 showsa system structure. Referring to FIGS. 1, 9, and 16, the code switchingaccording to the present embodiment will be explained below.

[0123] As stated in Embodiment 2, it is sometimes necessary to switchcodes during the communication. The code switching during thecommunication should be simultaneously performed at the base station 2and the mobile station 3 in order not to stop the communication and soas to reduce interfering in other communication. The simultaneousswitching is performed based on code switch timing information sent froma timing information sending unit 33 in the base station 2 to the mobilestation 3. FIG. 9 illustrates procedures of sending the code switchtiming information from the base station 2 to the mobile station 3.

[0124] The procedures, from transmitting a radio channel set-up requestmessage by the mobile station #2 (3 b) up to transmitting a codeswitching request message to the mobile station #1 (3 a) by the basestation controller 1, are the same as Embodiment 2 shown in FIG. 5.Therefore, procedures after the above will now be described in thepresent embodiment. A code switch informing unit 31 of the base stationcontroller 1 transmits a code switching request message indicating atarget code (code after switching) to the mobile station #1 (3 a). Onreceiving the code switching request message, a code switching unit 35of the mobile station #1 (3 a) starts preparing for the code switchingand transmits a code switching response message to the code switchinforming unit 31 of the base station controller 1.

[0125] After receiving the code switching response message, the codeswitch informing unit 31 transmits the message indicating that the codeof the mobile station #1 (3 a) can be switched, to the base station 2 a.Receiving the code switching response message, the timing informationsending unit 33 of the base station 2 a transmits a code switching startmessage to the mobile station #1 (3 a). This code switching startmessage includes start timing information for the code switching. Then,if the radio channel is composed of frames, the code switching startmessage can be time information per frame.

[0126] It is acceptable to switch at the time of frame N (N is equal to1 or an integer more than 1) after receiving the code switching startmessage. It is also acceptable for the timing information to use a codeperiod of the spreading code. Thus, a switching unit 34 is synchronizedwith the code switching unit 35 in order to switch the code.

[0127] Receiving the code switching start message, the code switchingunit 35 of the mobile station #1 (3 a) switches the original code (codebefore switching) to the target code (code after switching) based on thetime prescription specified in advance, using the timing information inthe message. Then, the code switching unit 35 transmits a code switchingcomplete message to the timing information sending unit 33 in the basestation 2 a using the target code. By receiving the code switchingcomplete message, the timing information sending unit 33 judges that thecode switching has been properly completed, and transmits the codeswitching complete message to the base station controller 1. Byreceiving the code switching complete message, a code releasing unit 32of the base station controller 1 judges that the code used by the mobilestation #1 (3 a) has been released, and transmits a radio channel set-upcomplete message for assigning the code used by the mobile station #1 (3a) to the mobile station #2 (3 b).

[0128] As the code switch timing between the mobile station 3 and thebase station 2 can be set up according to the present embodiment, it isnot necessary for the base station 2 to simultaneously perform receptionand transmission using both the original code and the target code.Consequently, interfering with other communications is reduced.

[0129] Embodiment 4.

[0130]FIG. 10 illustrates a code assignment sequence showing a codeswitching procedure using the maximum ratio combination of the mobilestation 3, according to the present embodiment. FIG. 17 shows a systemstructure. Referring to FIGS. 10 and 17, the code switching according tothe present embodiment will now be described.

[0131] The procedures, from transmitting the radio channel set-uprequest message by the mobile station #2 (3 b) up to transmitting thecode switching request message by the base station 2 a to the basestation controller 1, are the same as Embodiment 2 shown in FIG. 5.Therefore, procedures after the above will now be described in thepresent embodiment.

[0132] As shown in FIG. 10, after the base station 2 a sends the codeswitching request message to the base station controller 1, atransmitting unit 43 of the base station 2 a performs reception andtransmission with a regenerating unit 44 of the mobile station #1 (3 a)using both the original code (code before switching) and the target code(code after switching). When the base station controller 1 receives thecode switching request message from the base station 2 a, a first codeswitching unit 41 of the base station controller 1 transmits the messageto the mobile station #1 (3 a), and instructs the regenerating unit 44of the mobile station #1 (3 a) to switch the code. On receiving the codeswitching request message, the regenerating unit 44 begins to receivesignals based on the maximum ratio combination, using both the originalcode (code just in use) and the target code appointed by the codeswitching request message.

[0133] Regarding the transmission, either of the following methods isacceptable. One is to independently switch the code by using a secondcode switching unit 45 of the mobile station #1 (3 a). The other is totransmit signals from the mobile station #1 (3 a) with using both theoriginal code in use and the target code. In this case of transmittingwith using both the codes, it is also acceptable to perform the maximumratio combination at the base station 2.

[0134] When the second code switching unit 45 of the mobile station #1(3 a), in which the reception has been performed by the regeneratingunit 44 based on the maximum ratio combination, begins to independentlyreceive signals using the target code only, a switching completeinforming unit 46 of the mobile station #1 (3 a) transmits a codeswitching complete message to a code releasing unit 42 of the basestation controller 1 with using the target code. Then, the codereleasing unit 42 judges that the code having been used in the mobilestation #1 (3 a) until this time is released and the code switching iscompleted. The code releasing unit 42 transmits the code switchingcomplete message to the base station 2 a. On receiving the codeswitching complete message from the base station controller 1, the basestation 2 a stops transmitting signals to the mobile station #1 (3 a)using the original code, releases the original code, prepares for thereception and the transmission with the mobile station #2 (3 b) usingthe original code, and transmits a radio channel set-up complete messageto the base station controller 1.

[0135] After receiving the radio channel set-up complete message fromthe base station 2 a, the base station controller 1 transmits themessage indicating to set up the original code, having been used in themobile station #1 (3 a), for the mobile station #2 (3 b). As statedabove, the code switching process using the maximum ratio combinationhas been performed.

[0136] According to the present embodiment procedures, the code switchtiming can be achieved based on own timing of the mobile station 3without deteriorating the channel quality. Namely, the time needed forcode switching can be reduced.

[0137] In addition, it is also acceptable that the code assignment iscontrolled by the base station 2 instead of the base station controller1.

[0138] Having thus described several particular embodiments of theinvention, various alterations, modifications, and improvements willreadily occur to those skilled in the art. Such alterations,modifications, and improvements are intended to be part of thisdisclosure, and are intended to be within the spirit and scope of theinvention. Accordingly, the foregoing description is by way of exampleonly, and not intended to be limiting. The invention is limited only asdefined in the following claims and the equivalents thereto.

What is claimed is:
 1. A mobile station used for a radio communicationsystem employing CDMA (Code Division Multiple Access) for ratio accessand providing multi-rate transmission, the mobile station comprising aswitching unit for receiving code information for switching a firstcode, having been used, to a second code, receiving timing informationregarding timing of switching the first code to the second code, andswitching the first code to the second code based on the codeinformation and the timing information received, wherein the timinginformation is used for switching the first code to the second code at abase station.
 2. A radio communication method of mobile station used fora radio communication system employing CDMA (Code Division MultipleAccess) for ratio access and providing multi-rate transmission, theradio communication method comprising: receiving code information forswitching a first code, having been used, to a second code, receivingtiming information regarding timing of switching the first code to thesecond code, and switching the first code to the second code based onthe code information and the timing information received, wherein thetiming information is used for switching the first code to the secondcode at a base station.